My goal is to avoid this, by enabling the feature for GSM. The only problem is, I haven't found a way to do it, yet.
WB-AMR can be changed via the *#0011# Service-menu, but this only affects the UMTS setting not the GSM one.
WB-AMR can also be set by the CSC's customer.xml, but I can't seem to find a variable to do it for GSM.
My guess would be directly editing the modem and its variables via ADB, the file system or something like this, if that's possible.
Do you have an idea where to start?